1. Field of the Invention
The present invention relates to a pneumatic tire that has a directional tread pattern. To be more specific, the invention relates to a pneumatic tire capable of balancing the driving stability on a dry road surface with the performance on snow.
2. Description of the Related Art
Various proposals have been made for a pneumatic tire having a directional tread pattern that corresponds to a designated rotational direction of the tire (see, for example, Japanese patent application Kokai publication Nos. Hei 5-286312, 2002-67623, and 2005-231430). A pneumatic tire with a directional tread pattern includes plural main grooves and plural lug grooves in the tread portion. Each plural main groove extends in the tire-circumferential direction, and each plural lug groove extends in the tire-width direction. In addition, each lug groove is inclined in a direction opposite to the rotational direction of the tire while extending from the tread-center side to either one of the shoulder sides. The pneumatic tire with such a construction can exhibit favorable water drainage and excellent driving stability on a dry road surface.
An attempt to improve the performance on snow makes a pneumatic tire with a directional tread pattern have a difficulty in balancing the performance on snow with the driving stability on a dry road surface. For example, although enhancing the performance on snow, an increase in the number of lug-groove elements in the tread portion has a harmful effect on the tread stiffness. Such low tread stiffness results in a low driving stability on a dry road surface. In addition, forming many narrow lug grooves, which are more likely to be stuffed with snow, cannot always lead to a sufficient effect of improving the performance on snow.